Optical glass fibers, apparatus and preparation using reactive vapor transport and deposition

ABSTRACT

A new method for preparing low loss multimode and monomode glass optical fibers which avoids casting or pouring the core and clad melts is disclosed. The new technique is based on a reactive-gas-transport approach which avoids contamination from absorbing impurities and scattering centers by reacting the glass melt with reactive gases which remove impurities and increase the refractive index of the fiber.

BACKGROUND OF THE INVENTION

This invention pertains to a process for producing glass fibers and moreparticularly to a process for producing fluoride glass fibers using areactive vapor transport and deposition process.

At present, the casting approach is the only technique known to be usedin the fabrication of glass optical fibers. The Built-In Casting processdeveloped by Nippon Telegraph and Telephone Public Corp., Mitachi etal., Electron Lett. 17 (1981) 591, consists of pouring the claddingglass melt into a mold and then upsetting the mold to produce a claddingtube. The core melt is subsequently cast into the tube thus forming apreform. The Rotational Casting technique, developed at the NavalResearch Laboratory, Tran et al., Electron Lett. 18 (1982) 657, usesrotation of the mold to produce a highly concentric and uniform glasspreform. Another casting technique used in the fabrication ofpolymer-clad fluoride glass fibers consists of pouring the fluoride meltinto a cylindrical mold to form a glass rod. The rod is then jacketedwith a lower index polymer tubing such as Teflon prior to being drawninto fibers, Tran et al., Electron Lett. 19 (1983) 165.

All fluoride glass optical fibers prepared from the casting approachexhibit a wavelength independent scattering loss ranging from 5 dB/km toseveral hundred dB/km. Examination of these fibers revealed theformation of microcrystallites at the core-clad interface as well aslarge density fluctuations, striae, and bubbles in the bulk of thefiber. All of these scattering defects can be attributed to thermalvariations and uneven cooling of core and clad melts which are inherentin the casting process.

U.S. Pat. No. 4,414,012 to Suto et al. discloses a process whereby finesilica powders are doped using a reactive gas containing an easilyoxidizable compound for producing the dopane, an easily oxidizablesilicon compound and oxygen or water vapor. The finished particles maythen be used to form optical fibers. U.S. Pat. No. 4,341,873 to Robinsonet al. discloses the production of a fluorozirconate glass doped withchlorine by chemical vapor deposition.

U.S. Pat. No. 4,334,903 to MacChesney et al. relates to the productionof a silicon glass optical fiber having a graded index profile bychemical vapor deposition of reactant gases, varying the mixture ofdopants and glass forming materials with the deposition of successivelayers. U.S. Pat. No. 4,242,375 to Shiraishi et al. discloses thereaction of SiF₄ and H₂ O on a heated silica optical fiber substrate toproduce a new layer of silicon doped with fluorine. U.S. Pat. No.3,718,383 to Moore discloses the diffusion of an organic material into aplastic element to form a plastic optical element having a refractiveindex gradient.

The casting techniques and the other methods disclosed in the referencesdo not overcome the problem of scattering loss caused to flaws in thefibers. A technique is, therefore, needed which can supress thescattering defects which are generally observed in glass optical fibersproduced using these techniques.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodfor producing glass fibers which will reduce scattering losses due toflaws and impurities in the fibers.

It is another object of the present invention to provide a reactivevapor transport and deposition process for reducing scattering losses.

It is a further object of the present invention to provide a glass fiberwith a high refractive index.

These and other objects are achieved by passing reactive gases through arotating mold containing molten glass. The gases react with the innerwall of the molten glass to remove impurities that cause scatteringlosses and increase the refractive index of the resulting fiber.

In the preferred embodiment, fluoride glass starting materials areplaced in a rotating mold and the temperature is raised sufficiently tomelt the glass material. The mold is rotated at a speed sufficient tocause the glass melt to form a hollow glass tube inside the mold.Reactive gases are passed through the mold to purge the material ofwater, hydroxides and other impurities. After the impurities have beenremoved, reactive gases are blown through the mold to react with theglass material and reduce the refractive index on the inner wall of theglass tube. The mold is cooled and a preform, which can later be drawninto glass fibers, is formed from the resulting material.

Other objects, advantages, and novel features of the present inventionwill become apparent from the following detailed description of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the apparatus used for the ReactiveVapor Transport Process (RVT).

FIG. 2 is a cross-sectional view of an RVT tube preform formed accordingto the present invention.

FIG. 3 is a graph showing the index profile of a RTV tube preform.

FIG. 4 is a cross-sectional view of the apparatus used for the ReactiveVapor Transport Deposition Process (RVTD).

DETAILED DESCRIPTION OF THE INVENTION

A new method for preparing low loss multimode and monomode glass opticalfibers which avoids casting or pouring the core and clad melts isdisclosed. The new technique is based on a reactive-gas-transportapproach which avoids contamination from absorbing impurities andscattering centers such as bubbles, dust particles, microcrystals, phaseseparation, striae, and core-clad defects which are generally formedupon casting the glass melt. A diagram of the newly developed ReactiveVapor Transport approach (RVT) is shown in FIG. 1. It consists of arotating mold 10 which contains the glass melt 12. The mold is made outof platinum, gold, graphite, vitreous carbon, or other suitablematerial, and is equipped with gas entry 22 and exit 14 ports. All glasssystems with a viscosity equal to or less than 10 poises at or above thecrystallization temperature can be cast using the present rotationalcasting process. In particular, the process can be used with fluoride,chloride, bromide, fluorophosphate, and mixed halide glasses. Fluoride,chloride, and bromide glasses are preferred with fluroide glasses beingmost preferred. The rotating melt is heated to between 500° C. and 1000°using heating rods 16 and rotated at a rate sufficient to form a hollowtube inside the mold. When the melt is homogenized and refined, it ispurged with one or more reactive gases 18 to remove hydroxides molecularwater, and other impurities present in the melt. Any reactive gas ormixture of gases which can remove the impurities can be used in thepresent process. SF₆, NF₃, F₂, and CF₄ are preferred with SF₆ being mostpreferred. Subsequently, one or more reactive gases 20 are blown throughpast the melt and are allowed to diffuse, mix, and react with thesurface of the molten glass. Any reactive gas or mixture of gases whichcan react with the glass to produce a higher refractive index can beused in the present process. HCl, HBr, HI, Cl₂, Br₂, I₂, SnI₂ +Ar, BiI₃+Ar, SnBr₂ +Ar, BiBr₃ +Ar, BiF₃ +Ar, PF₅ +Br₂, PF₅ +Cl₂ are preferredwith HCl, HBr, HI, Cl₂, Br₂, and I₂ being most preferred. The gas mustbe selected such that the halogen in the gas does not correspond to thehalogen in the glass, i.e. HCl gas cannot be used for chloride glasses.Also, the halogen in the gas should have a higher molecular weight thanthe halogen in the glass, i.e. any of the gases listed can be used withfluoride glasses while only gases with bromine or iodine can be usedwith chloride glasses. The resulting reaction increases the refractiveindex to the inner wall of the melt. After the core region is formed,the rotating mold is rapidly quenched to the glass transitiontemperature using a metal brushing or other suitable means. Theresulting tube preform can then be drawn into fibers.

A cross-sectional view of an RVT tube preform is shown in FIG. 2. Thethickness of the core can be readily controlled by diffusion and mixingparameters such as gas flow rates, melt temperature, exchange rate,quenching rate, and mold heat capacity. The extent of the increase inindex of refraction can also be controlled by the nature of the reactivegases used, and the exchange time and temperature.

To show feasibility of the reactive gas transport approach, ion-exchangeexperiments where carried with ZrF₄ -BaF₂ -LaF₃ -AlF₃ -LiF melts and thefollowing reactive gases or mixture of gases: HCl, HBr, HI, Cl₂, Br₂,I₂, SnI₂ +Ar, BiI₃ +Ar, SnBr₂ +Ar, BiBr₃ +Ar, BiF₃ +Ar, PF₅ +Br₂, PF₅+Cl₂. The refractive index profile of a RVT tube thus obtained is shownin FIG. 3 where the difference in indices n= ni-n(b)! is plotted againstb; where ni represents the measured index at the inner wall of the tubeand b is the overall tube thickness. The small core and the parabolicindex profile thus obtained make possible the fabrication of both singlemode and graded-index glass fibers by RVT processing. The results alsoshowed that substantial increase in refractive index (n) was obtainedwith chlorine and bromine compounds. Namely, n as high as 0.01 wasobtianed when the base glass was treated with PF₅ +Br₂, PF₅ +Cl₂, HBr,or HCl at 1000° C. for less than 5 min; and no crystals and densityvariations were observed in the treated glass samples. Furthermore, theaddition of PF₅ to the base fluoride glass has drastically increased theglass stability; namely, an increase of 20° C. in the glass workingrange was observed.

The feasibility of forming a core inside a glass tube using the ReactiveVapor Transport process has allowed, the the first time, theincorporation of a complete vapor deposition approach in the preparationof glass preforms termed Reactive Vapor Transport and Deposition process(RVTD). This vapor process will essentially minimize the fiberabsorption losses. A diagram of the RTVD is shown in FIG. 4. Startingmaterials with high vapor pressure or low sublimation temperatures arestored in bubblers 44; these starting chemicals are derived from metalchlorides, metal bromides, metal iodines, and organo-metallic compoundssuch as ZrCl₄, ZrCl₂, ZrCl₃, ZrBr₂, ZrBr₃, ZrBr₄, ZrI₄, Ba (C₁₀ H₁₉ O₂),BaI₂, AlCl₃, AlBr₃, Al (sec-butoxide), LaI₃, PbBr₂, and LiNH₂. Properamounts of the starting chemicals are sublimed by heating bubblers 44and are carried inside a cold rotating mold using Argon or othersuitable inert gas as a carrier gas. The cold mold cools the materialand causes it to be deposited om the inner surface of the mold. Theabsorbing impurities having very low vapor pressure will stay behind inthe bubblers. Subsequently, the raw materials are fluorinated withflowing HF contained in bubbler 46. When the halogenation step iscompleted, the mold temperature is raised to 500° C.-1000° C. formelting. And finally, the RVTD technique described earlier is used toproduce the core.

The invention having been generally described, the following examplesare given as particular embodiments of the invention and to demonstratethe practice and advantages thereof. It is understood that the examplesare given by way of illustration and are not intended to limit thespefication or the claims to follow in any manner.

EXAMPLE I

25 g of Fluoride glass of composition 53ZrF₄ -19BaF₂ -5LaF₃ -3AlF₃ -20LiF (in mole %) were melted in a platinum crucible, in a resistancefurnace, under an Ar atmosphere. The argon flowrate was set at 6 SCFH.The melt was soaked at 850° C. for 45 minutes. The melt was cast into abrass mold and rotated at 2000 rpm. The brass mold was preheated to 250°C. Immediately a stream of gas mixture consisting of 2/3 Ar and 1/3 HClwas introduced into the rotating glass to activate the ion-exhangeprocess. The Ar-HCl mixture flowrate was set at 1 SCFH. After 4 minutes,the Ar-HCl flow was stopped, and the glass tube was purged for 2 minuteswith Ar. The tube was annealed at 250° C. for 2 hrs.

EXAMPLE II

25 g of Fluoride glass of composition 53ZrF₄ -19BaF₂ -5LaF₃ -3AlF₃ -20LiF (in mole %) were melted in a platinum crucible, in a resistancefurnace, under an Ar atmosphere. The argon flowrate was set at 6 SCFH.The melt was soaked at 850° C. for 45 minutes. The melt was cast into abrass mold and rotated at 2000 rpm. The brass mold was preheated to 250°C. Immediately a stream of gas mixture consisting of 2/3 Ar and 1/3 HClwas introduced into the rotating glass to activate the ion-exhangeprocess. The Ar-HCl mixture flowrate was set at 1 SCFH. After 1 minute,the Ar-HCl flow was stopped, and the glass tube was purged for 2 minuteswith Ar. The tube was annealed at 250° C. for 2 hrs.

EXAMPLE III

25 g of Fluoride glass of composition 53ZrF₄ -19BaF₂ -5LaF₃ -3AlF₃ -20LiF (in mole %) were melted in a platinum crucible, in a resistancefurnace, under an Ar atmosphere. The argon flowrate was set at 6 SCFH.The melt was soaked at 850° C. for 45 minutes. The melt was cast into abrass mold and rotated at 2000 rpm. The brass mold was preheated to 250°C. Immediately a stream of gas mixture consisting of 2/3 Ar and 1/3 HClwas introduced into the rotating glass to activate the ion-exhangeprocess. The Ar-HCl mixture flowrate was set at 1 SCFH. After 30seconds, the Ar-HCl flow was stopped, and the glass tube was purged for2 minutes with Ar. The tube was annealed at 250° C. for 2 hrs.

EXAMPLE IV

25 g of Fluoride glass of composition 53ZrF₄ -19BaF₂ -5LaF₃ -3AlF₃ -20LiF (in mole %) were melted in a platinum crucible, in a resistancefurnace, under an Ar atmosphere. The argon flowrate was set at 6 SCFH.The melt was soaked at 850° C. for 45 minutes. The melt was cast into abrass mold and rotated at 2000 rpm. The brass mold was preheated to 250°C. Immediately a stream of gas mixture consisting of 2/3 Ar and 1/3 HBrwas introduced into the rotating glass to activate the ion-exhangeprocess. The Ar-HBr mixture flowrate was set at 1 SCFH. After 4 minutes,the Ar-HBr flow was stopped, and the glass tube was purged for 2 minuteswith Ar. The tube was annealed at 250° C. for 2 hrs.

EXAMPLE V

25 g of Fluoride glass of composition 53ZrF₄ -19BaF₂ -5LaF₃ -3AlF₃ -20LiF (in mole %) were melted in a platinum crucible, in a resistancefurnace, under an Ar atmosphere. The argon flowrate was set at 6 SCFH.The melt was soaked at 850° C. for 45 minutes. The melt was cast into abrass mold and rotated at 2000 rpm. The brass mold was preheated to 250°C. Immediately a stream of gas mixture consisting of 2/3 Ar and 1/3 HIwas introduced into the rotating glass to activate the ion-exhangeprocess. The Ar-HI mixture flowrate was set at 1 SCFH. After 4 minutes,the Ar-HI flow was stopped, and the glass tube was purged for 2 minuteswith Ar. The tube was annealed at 250° C. for 2 hrs.

EXAMPLE VI

25 g of Fluoride glass of composition 53ZrF₄ -19BaF₂ -5LaF₃ -3AlF₃ -20LiF (in mole %) were melted in a platinum crucible, in a resistancefurnace, under an Ar atmosphere. The argon flowrate was set at 6 SCFH.The melt was soaked at 850° C. for 45 minutes. The melt was cast into abrass mold and rotated at 2000 rpm. The brass mold was preheated to 250°C. Immediately a stream of gas mixture consisting of 2/3 Ar and 1/3 Br₂was introduced into the rotating glass to activate the ion-exhangeprocess. The Ar-Br₂ mixture flowrate was set at 1 SCFH. After 4 minutes,the Ar-Br₂ flow was stopped, and the glass tube was purged for 2 minuteswith Ar. The tube was annealed at 250° C. for 2 hrs.

EXAMPLE VII

25 g of Fluoride glass of composition 53ZrF₄ -19BaF₂ -5LaF₃ -3AlF₃ -20LiF (in mole %) were melted in a platinum crucible, in a resistancefurnace, under an Ar atmosphere. The argon flowrate was set at 6 SCFH.The melt was soaked at 850° C. for 45 minutes. The melt was cast into abrass mold and rotated at 2000 rpm. The brass mold was preheated to 250°C. Immediately a stream of gas mixture consisting of 2/3 Ar and 1/3 I₂was introduced into the rotating glass to activate the ion-exhangeprocess. The Ar-I₂ mixture flowrate was set at 1 SCFH. After 4 minutes,the Ar-I₂ flow was stopped, and the glass tube was purged for 2 minuteswith Ar. The tube was annealed at 250° C. for 2 hrs.

EXAMPLE VIII

25 g of Fluoride glass of composition 53ZrF₄ -19BaF₂ -5LaF₃ -3AlF₃ -20LiF (in mole %) were melted in a platinum crucible, in a resistancefurnace, under an Ar atmosphere. The argon flowrate was set at 6 SCFH.The melt was soaked at 850° C. for 45 minutes. The melt was cast into abrass mold and rotated at 2000 rpm. The brass mold was preheated to 250°C. Immediately a stream of gas mixture consisting of 2/3 Ar and 1/3 Cl₂was introduced into the rotating glass to activate the ion-exhangeprocess. The Ar-Cl₂ mixture flowrate was set at 1 SCFH. After 4 minutes,the Ar-Cl₂ flow was stopped, and the glass tube was purged for 2 minuteswith Ar. The tube was annealed at 250° C. for 2 hrs.

The Reactive Vapor Transport process disclosed is used to suppress thenumerous scattering defects which are generally observed in cast glasspreforms, particularly fluoride glass preforms. The new process has alsopermitted the graded-index profiling in glass fibers, the preparation ofsingle mode glass fibers, and the prevention and removal of hydroxidecontamination by reactive gases. Furthermore, this new process can beextended to a complete vapor phase process using the Reactive VaporTransport and Deposition technique, thus allowing production of glassfibers free from contamination by absorbing impurities.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent by theUnited States is:
 1. A process for the production of glass fibers, thesteps of which comprise;placing a glass fiber starting material into arotating mold; heating said glass fiber starting material in said moldto a temperature sufficient to cause said material to melt; rotatingsaid mold at a rate sufficient to cause said starting material to form ahollow tube inside said mold; purging said glass fiber starting materialwith one or more reactive gases to remove hydroxides, water, and otherimpurities; blowing one or more reactive gases through said mold, saidgases reacting with said melt to increase the refractive index; rapidlyquenching said mold to the glass transition temperature to form a glassfiber preform; and drawing said preform into a glass fiber.
 2. Theprocess of claim 1 wherein the step of placing said glass fiber startingmaterial into said rotating mold comprises placing said glass fiberstarting material selected from the group consisting of fluoride,chloride, bromide, fluorophosphate, and mixed halide glasses into saidmold.
 3. The process of claim 2 wherein the step of placing said glassfiber starting material into said rotating mold comprises placing saidglass fiber starting material selected from the group consisting offluoride, chloride, bromide glasses into said mold.
 4. The process ofclaim 3 wherein the step of placing said glass fiber starting materialinto said rotating mold comprises placing fluoride glass into said mold.5. The process of claim 1 wherein said heating step comprises heatingsaid glass fiber starting material to a temperature between about500°-1000°.
 6. The process of claim 1 wherein said purging stepcomprises purging said glass fiber starting material with reactive gasesselected from the group consisting of SF₆, NF₃, F₂, and CF₄.
 7. Theprocess of claim 2 wherein said blowing step comprises blowing reactivegases selected from the group consisting of HCl, HBr, HI, Cl₂, Br₂, I₂,SnI₂ +Ar, BiI₃ +Ar, SnBr₂ +Ar, BiBr₃ +Ar, BiF₃ +Ar, PF₅ +Br₂, and PF₅+Cl₂ through said mold.
 8. The process of claim 7 wherein said saidblowing step comprises blowing reactive gases selected from the groupconsisting of HCl, HBr, HI, Cl₂, Br₂, and I₂ through said mold.
 9. Aprocess for the production of glass fibers, the steps of whichcomprise;heating a glass fiber starting material to a temperaturesufficient to cause said material to sublime; transporting said materialto a cold rotating mold using an inert carrier gas; cooling saidmaterial thereby causing said material to deposit on the inner surfaceof said mold; halogenating said materials to produce a glass fibermaterial; heating said material to a temperature above said glass fibermaterials melting point; purging said material with reactive gases;blowing one or more reactive gases through said mold, said gasesreacting with said melt to increase the refractive index; rapidlyquenching said mold to the glass transition temperature to form a glassfiber preform; and drawing said preform into a glass fiber.
 10. Theprocess of claim 9 wherein said heating step comprises heating saidglass fiber starting material selected from the group consisting ofmetal chlorides, metal bromides, metal iodines, and organo-metalliccompounds.
 11. The process of claim 10 wherein said heating stepcomprises heating said glass fiber starting material selected from thegroup consisting of such as ZrCl₄, ZrCl₂, ZrCl₃, ZrBr₂, ZrBr₃, ZrBr₄,ZrI₄, Ba (C₁₀ H₁₉ O₂), BaI₂, AlCl₃, AlBr₃, Al (sec-butoxide), LaI₃,PbBr₂, and LiNH₂.
 12. The process of claim 9 wherein said heating stepcomprises heating said glass fiber starting material to a temperaturebetween about 500°-1000°.
 13. The process of claim 9 wherein saidhalogenating step comprises reacting said glass fiber starting materialwith a halogen gas, said halogen in said halogen gas having a highermolecular weight than the halogen in the glass.
 14. The process of claim9 wherein said purging step comprises purging said glass fiber startingmaterial with reactive gases selected from the group consisting of SF₆,NF₃, F₂, and CF₄.
 15. The process of claim 9 wherein said blowing stepcomprises blowing reactive gases selected from the group consisting ofHCl, HBr, HI, Cl₂, Br₂, I₂, SnI₂ +Ar, BiI₃ +Ar, SnBr₂ +Ar, BiBr₃ +Ar,BiF3+Ar, PF₅ +Br₂, and PF₅ +Cl₂ through said mold.
 16. The process ofclaim 15 wherein said blowing step comprises blowing reactive gasesselected from the group consisting of HCl, HBr, HI, Cl₂, Br₂, and I₂through said mold.
 17. An apparatus for producing glass optical fibers,comprising:a rotating mold for containing a glass melt, said mold havinggas entry and gas exit ports; means for rotating said mold; means forheating said mold to a temperature sufficient to produce said glassmelt; and means for supplying gases to said mold through said gas entryport.
 18. The apparatus of claim 17 wherein said rotating mold is madefrom a material selected from the group consisting of platinum, gold,graphite, or vitreous carbon.
 19. The apparatus of claim 17 wherein saidmeans for heating said mold is heating rods, said heating rods beingcapable of heating said mold to a temperature between 500°-1000° C. 20.The apparatus of claim 19 wherein said means for supplying gases to saidmold comprises a plurality of bubblers from which reactive gases orliquid vapors and glass optical fiber starting materials can betransported to said mold by a carrier gas, said bubblers having aheating means for controlling the temperature of said bubblers.
 21. Theapparatus of claim 20 wherein said heating means for controlling thetemperature of said bubblers is selected from the group consisting ofheating tape and heating mattles.
 22. The product of the process ofclaim
 1. 23. The product of the process of claim 9.